Materials of Construction - Wet Electrostatic Precipitators
December 12, 2008

Introduction Lead Plastic/FRP Stainless Steel and Alloys

Associated Links Gas Cleaning System - Wet Electrostatic Precipitators Metals - Lead Metals - Alloy C-276 Operation Maintenance and Inspection Suppliers Erection Testing

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Introduction

Each acid plant is unique with its own set of operating conditions.  One material of construction that is suitable in one plant may be totally unsuitable in another similar plant.  The only difference may be a contaminant that is present in one plant and not the other.  The selection of the proper material of construction must be based on research, experience, testing and a little bit of luck sometimes.

The materials of construction available for WESP’s range from the traditional lead and lead lined construction to plastic/FRP, stainless steel and combinations of all these materials.  The selection of the material of construction must take into account its chemical resistance (e.g. against H2SO4, chlorine, fluorine, mercury compounds, etc.), its mechanical and electrical properties.

Lead

There are still many of the traditional lead/lead-lined units in operation providing satisfactory operation and cleaning efficiency, however, as these units age, maintenance costs begin to increase significantly.  Workers skilled in lead burning are becoming more difficult to obtain when building a new unit or undertaking repairs to an existing unit.  The health concerns connected with exposure to and working with lead are well known.

WESP’s constructed primarily of lead are still available but are becoming less common, particularly in new construction.

Lead and lead lining are still being used in selected components within the WESP, such as, upper and lower support grids, discharge electrodes and insulator compartments.

The advantage of lead as a material of construction is its chemical resistance to the typical gases and liquids present in an acid plant gas cleaning system.

Plastic/FRP

Plastics and FRP have been used extensively as a material of construction for WESP’s as early as 1970.  A WESP constructed from plastic/FRP is light and compact, which reduces installation cost substantially.

Plastics and FRP have the advantage that they are resistant to most of the compounds that will be typically present in the gas or liquid.  FRP will require a synthetic corrosion barrier (e.g. Nexus Veil) if there are fluorides present in the gas or liquid.  As well, additives must be added to FRP to make it UV resistant.

One major problem with plastic/FRP materials is they are generally non-conductive.  To get around this problem some design incorporate a conductive carbon veil in the collecting tubes surface or rely on a continuous thin liquid film on the collecting tube surface.

Stainless Steels and Alloys

Stainless steel and alloys are being used more extensively in sulphuric acid plants to address corrosion and maintenance problems.  WESP’s constructed of stainless steel were first placed into service in the 1970’s? 

The choice of stainless steel or alloy to be used will be primarily based on its corrosion resistance in the gas and liquid environment.  316L stainless steel is generally not used for wetted parts because of its limited corrosion resistance in the environment typical for an acid plant gas cleaning system.  It may be used in areas where it is not exposed to the process, such as structural components.

904L stainless steel (UNS N08904) is the minimum grade of stainless steel recommended depending again its corrosion resistance to the process fluids.  The use of UNS S31254 (i.e. 254 SMO) would be considered an upgrade from 904L SS when the conditions require a material with superior corrosion resistance.  When the conditions are very aggressive from a corrosion point of view, Alloy C-276 (UNS N10276) can be utilized.

With each upgrade in material comes an increase in price generally.